Seen that before, just ignore it, but OMV is mean't to be run headless so you wouldn't see any errors

It's not so a big issue for me at the moment, but I want to run the server not all of the day, but only when I'm at home. Therefore I want my openhabian2 raspberry pi to check, if my smartphone is at home. In case it is, it shall wake up the server on lan. I have to check if it then already is at a point, where the services,... of OMV can be used.
But if I get you right you say, this is no message that comes from OMV, as it runs headless?!

Have found the manual it appears that there maybe two settings that you have to change.

Boot List Options: I would set this to legacy
SATA Operation: default setting is Raid ON, I would set this to AHCI

If your Samsung Evo is connected to one of the SATA ports and you have made no changes then the above 2 options are set it would explain why OMV does not see the drives.
Changing either of those or both 'may' prevent OMV from starting, I say may, you will not know until you make any changes to the bios.

Option 1 you may be able to leave alone, but Option 2 will need changing to AHCI, otherwise you have to use the onboard software to add the two disks, but this would be presented to OMV as a single hard drive! Something you do not want to do.

If Option 2 is set to Raid ON this would explain why OMV cannot see them.

a short info regarding my problem: Yesterday in the evening I changed the BIOS setting "SATA Operation" to "AHCI". The "Boot List Options" was already set to "legacy".

What a pitty, this didn't make it and when I check the drives within the BIOS, it says SATA1 = empty or none, SATA2 = ... Only one SATA seems to be detected as connected and that's with the DVD drive.

My next step will be, that I connect both drives with an external USB interface and directly add them to the USB port of my windows computer, so I at least see, if they are detected,... Because they are new (I unboxed them and immediately plugged them into the T30), I don't think both are damaged.

I bought the T30 second-hand (even if the seller said, he never used but only bought it), I'll also reset the BIOS settings to default, perhaps the first owner changed anything. This idea came just some minutes ago, so I didn't try that out, yet.

The fact that those settings were changed within the BIOS would imply the machine had been used, the second setting in particular as the default setting for that is RAID.

Testing the drives via a usb even though they are new makes sense at least you can confirm the drives are working.

With reference to the drives not being seen in the BIOS; Faulty controller ports, SATA Cables, Power connections. One option might be is to use the DVD port to test the drives and use the DVD to test the functionality of the other ports.
Boards are usually set up as SATA 0 1 2 3 etc with the DVD being plugged into SATA 0.

Resetting to defaults would not be something I would recommend especially option 2 as this will initiate RAID, changing option 1 would prevent the Samsung from booting.